This invention relates to apparatus and method for on-line measuring of cassettes and similar devices used for handling semiconductor wafers to determine whether such devices are warped or otherwise have been distorted dimensionally during use and if the wafers are within close dimensional tolerances for flatness and shape.
In the manufacture of semiconductors, such as integrated circuits (ICs), dynamic random access memories (DRAMs), etc., large thin wafers (typically of silicon) from which the semiconductors are fabricated must frequently be transferred from one processing location to another. This transfer of wafers must be carried out under clean conditions. It is the usual practice to load a number of wafers (e.g., several dozen) into a xe2x80x9ccassettexe2x80x9d so that they can be carried under clean-room conditions safely and efficiently from one piece of equipment to another, or processed in situ in the cassette. In one example the wafers are vertically spaced apart in the cassette by horizontal grooves or slots along the walls of the cassette which support opposite edges of each wafer and hold the wafers in precise locations. The wafers are fed by an automatic device of one kind or another, one-by-one into or out from their respective cassettes. It is important that each wafer be accurately held in its respective location within a cassette since an automated wafer-loading and unloading device relies on dimensions within a narrow range of tolerances in determining where to find a wafer within a cassette, or where to put the wafer. Maintenance of the dimensional accuracy and continuing structural integrity of a cassette are highly important.
Cassettes, depending on their usage, are made of materials, such as plastic, metal, quartz, or ceramic, having varying degrees of dimensional stability. In some instances wafers are processed in situ in their cassettes (e.g., during chemical cleaning and rinsing), and as a consequence the cassettes are exposed to liquids, chemical agents, large changes in temperature, etc. These factors can induce stresses in the cassettes, which stresses in turn can result in dimensional changes, warpage, etc. beyond design tolerances. When this happens wafers being inserted into or removed from a cassette can be damaged, chipped, or scratched by the automated device being used, or by the cassette itself. This in turn can leave loose fragments or chips of a wafer in the cassette to contaminate other wafers. It is desirable therefore to be able to monitor on a continuing basis the dimensional integrity, degree of warpage, etc. of cassettes and wafers as they are put through semiconductor production processes, and to detect out-of-tolerance conditions of the cassettes or their wafers before damage (or further damage) results.
In one embodiment the present invention is an apparatus comprising a camera capable of taking an image of an object, and providing at an output thereof, a digital output representative of the object and a computer. The computer has an input coupled to the output of the camera and comprises a memory capable of storing data denoting nominal dimensions of the object, and further comprises a comparator for comparing the data from the image of the object received from the camera with that stored in the memory to determine if the object has dimensions which are within a predetermined acceptable variation of the nominal dimensions.
In a second embodiment the present invention is an apparatus for on-line monitoring of objects such as cassettes and similar devices used for handling semiconductor wafers to determine whether the objects remain within relevant dimensional tolerances during wafer processing. The apparatus comprises a digital camera for photographing an object at suitable distance to provide digital images to be used to determine whether relevant dimensions of the object are within tolerance limits, the object and the camera being moved relative to each other to position them for taking pictures of one or more faces of the object, and a computer coupled to the camera for storing images of the object taken by the camera and for analyzing the images to determine whether relevant dimensions of the object are still within tolerance limits.
In a third embodiment the present invention is an apparatus for on-line monitoring of objects such as cassettes and similar devices used for handling semiconductor wafers to determine whether the objects remain within relevant dimensional tolerances during wafer processing. The apparatus comprises digital camera means for photographing an object at suitable distance to provide digital images to be used to determine whether relevant dimensions of the object are within tolerance limits, means for moving the object and the camera relative to each other to position them for taking pictures of one or more faces of the object, and a computer. The computer is coupled to the means for moving and to the camera for storing images of the object taken by the camera and for analyzing the images to determine whether relevant dimensions of the object are still within tolerance limits.
In a fourth embodiment the present invention is a method for comparing dimensions of an object and determining if the dimensions are within predetermined acceptable variations from nominal dimensions of the object. The method comprises the steps of: taking an image of the object; and inputting the image of the object to a computer which comprises a memory capable of having stored therein nominal dimensions of the object, and a comparator capable of comparing the stored nominal dimensions of the object with the data contained in the image of the object so as to determine if the actual dimensions of the object are within predetermined acceptable variations from the nominal dimensions of the object.
In a fifth embodiment the present invention is a method for continual monitoring of structural integrity of objects such as cassettes and related devices used in handling semiconductor wafers during the processing thereof. The method comprises the steps of: obtaining digital images of relevant surfaces and faces of an object; storing the digital images in computer memory; analyzing the digital images to obtain relevant dimensional data of the object; and comparing the dimensional data of the object with corresponding ideal dimensional data of the object to determine whether the dimensions of the object are still within acceptable limits or tolerances.
In a sixth embodiment the present invention is a method for measuring relevant dimensions of objects such as cassettes and related devices used in handling semiconductor wafers during the processing thereof. The method comprises the steps of: obtaining digital images of surfaces and faces of an object, the images having sufficient resolution to detect dimensional changes of a few microns; storing the digital images in computer memory; analyzing the digital images to obtain relevant dimensional data of the object; and determining whether the dimensional data of the object lie within acceptable limits of variation of the dimensions of the object.